Cell confluence regulates claudin-2 expression: possible role for ZO-1 and Rac.

Claudin-2 (Cldn-2) is a channel-forming tight junction (TJ) protein in the proximal tubules that mediates paracellular Na+ transport and has also emerged as a regulator of proliferation and migration. Expression of Cldn-2 is altered by numerous stimuli, but the underlying mechanisms remain incompletely understood. Here we show that Cldn-2 protein and mRNA expression were low in subconfluent tubular cells and increased during junction maturation. Cldn-1 or occludin did not exhibit similar confluence-dependence. Conversely, disruption of TJs by Ca2+ removal or silencing of zonula occludens-1 (ZO-1) or ZO-2 induced a large drop in Cldn-2 abundance. Immunofluorescent staining revealed a more uneven Cldn-2 staining in nascent, Cldn-1-positive TJs. Subconfluence and ZO-1 silencing augmented Cldn-2 degradation and reduced Cldn-2 promoter activity, suggesting that insertion into the TJs slows Cldn-2 turnover. Indeed, blocking endocytosis or lysosomal degradation increased Cldn-2 abundance. Cell confluence increased expression of the junctional adapters ZO-1 and -2, and the small GTPase Rac, and elevated Rac activity and p21-activated kinase (Pak) phosphorylation, suggesting that they might mediate confluence-dependent Cldn-2 regulation. Indeed, Rac silencing or Pak inhibition strongly reduced Cldn-2 protein abundance, which was likely the combined effect on turnover, as these interventions reduced Cldn-2 promoter activity and augmented Cldn-2 degradation. Taken together, our data suggest that TJ integrity and maturity, ZO-1 expression/TJ localization, and Rac/Pak control Cldn-2 degradation and synthesis. A feedback mechanism connecting Cldn-2 expression with junction remodeling, e.g., during wound healing, epithelial-mesenchymal transition, or tumor metastasis formation, may have important downstream effects on permeability, proliferation, and migration.

Tumor Necrosis Factor-α Increases Claudin-1, 4, and 7 Expression in Tubular Cells: Role in Permeability Changes.

Tumor necrosis factor-α (TNFα), is a pathogenic cytokine in kidney disease that alters expression of claudins in tubular cells. Previously we showed that in LLC-PK1 cells TNFα caused a biphasic change in transepithelial resistance (TER) consisting of an early drop and recovery, followed by a late increase. However, the underlying mechanisms and the role of specific claudins in the TER effect remained incompletely understood. Here we sought to define how TNFα affects claudins 1, 4, and 7 in tubular cells and to correlate their changes with the TER effect. We show that TNFα elevates total and surface levels of Cldn-1, 4, and 7, and increases their mRNA expression through the ERK and JNK pathways. Further, JNK is also important for TNFα-induced changes in claudin-2 expression. Continuous monitoring of TER using Electric cell-substrate impedance sensing (ECIS) reveals that the two phases of the TNFα effect are differently regulated. Specifically, inhibition of the ERK or JNK pathways prevent the late TER increase, but not the early TER effect. Silencing experiments also show that Cldn-1 is necessary for the early TNFα-induced TER change, while all three claudins appear to contribute to the late TER increase. In summary, we define a central role for ERK and JNK in TNFα-induced altered claudin expression and barrier tightening. Together, our current and previous works show that the TNFα-induced early TER effect requires claudin-1, while claudin-2 decrease is a significant mediator of the late TER increase, and elevation in claudin-1, 4, and 7 contribute to a smaller extent. J. Cell. Physiol. 232: 2210-2220, 2017. © 2016 Wiley Periodicals, Inc.

Tumor necrosis factor-α induces a biphasic change in claudin-2 expression in tubular epithelial cells: role in barrier functions.

The inflammatory cytokine tumor necrosis factor-α (TNF-α) is a pathogenic factor in acute and chronic kidney disease. TNF-α is known to alter expression of epithelial tight junction (TJ) proteins; however, the underlying mechanisms and the impact of this effect on epithelial functions remain poorly defined. Here we describe a novel biphasic effect of TNF-α on TJ protein expression. In LLC-PK1 tubular cells, short-term (1-6 h) TNF-α treatment selectively elevated the expression of the channel-forming TJ protein claudin-2. In contrast, prolonged (>8 h) TNF-α treatment caused a marked downregulation in claudin-2 and an increase in claudin-1, -4, and -7. The early increase and the late decrease in claudin-2 expression involved distinct mechanisms. TNF-α slowed claudin-2 degradation through ERK, causing the early increase. This increase was also mediated by the EGF receptor and RhoA and Rho kinase. In contrast, prolonged TNF-α treatment reduced claudin-2 mRNA levels and promoter activity independent from these signaling pathways. Electric Cell-substrate Impedance Sensing measurements revealed that TNF-α also exerted a biphasic effect on transepithelial resistance (TER) with an initial decrease and a late increase. Thus there was a good temporal correlation between TNF-α-induced claudin-2 protein and TER changes. Indeed, silencing experiments showed that the late TER increase was at least in part caused by reduced claudin-2 expression. Surprisingly, however, claudin-2 silencing did not prevent the early TER drop. Taken together, the TNF-α-induced changes in claudin-2 levels might contribute to TER changes and could also play a role in newly described functions of claudin-2 such as proliferation regulation.

New insights into functions, regulation, and pathological roles of tight junctions in kidney tubular epithelium.

Epithelial tight junctions (TJs) seal off the intercellular space to generate a paracellular barrier as well as a selective transport pathway. They also maintain apicobasal polarization of cells. In addition, TJs serve as signaling platforms that orchestrate many epithelial functions. The past years have brought about a remarkable increase in our understanding of the structure and function of TJs. The overall aim of this review is to provide an update on this new knowledge, with special emphasis on TJs in the kidney tubular epithelium. TJs are key determinants of tubular transport processes, contributing to ion and fluid homeostasis. They are also central organizers of the structure of the tubular epithelium and many aspects of tubular cell functions, beyond transport processes. Many pathological states in the kidney, including inflammation, acute kidney injury, and chronic progressive diseases affect the tubular TJs, with important consequences with regards to the pathogenesis and pathophysiology of various nephropathies. In fact, altered TJ structure and function are increasingly recognized as a key step in the generation of renal diseases. In this review, we provide an overview of the protein composition, function and regulation of TJs in the kidney tubules, as well as their involvement and role in diseases.

Central role of the exchange factor GEF-H1 in TNF-α-induced sequential activation of Rac, ADAM17/TACE, and RhoA in tubular epithelial cells.

Transactivation of the epidermal growth factor receptor (EGFR) by tumor necrosis factor-α (TNF-α) is a key step in mediating RhoA activation and cytoskeleton and junction remodeling in the tubular epithelium. In this study we explore the mechanisms underlying TNF-α-induced EGFR activation. We show that TNF-α stimulates the TNF-α convertase enzyme (TACE/a disintegrin and metalloproteinase-17), leading to activation of the EGFR/ERK pathway. TACE activation requires the mitogen-activated protein kinase p38, which is activated through the small GTPase Rac. TNF-α stimulates both Rac and RhoA through the guanine nucleotide exchange factor (GEF)-H1 but by different mechanisms. EGFR- and ERK-dependent phosphorylation at the T678 site of GEF-H1 is a prerequisite for RhoA activation only, whereas both Rac and RhoA activation require GEF-H1 phosphorylation on S885. Of interest, GEF-H1-mediated Rac activation is upstream from the TACE/EGFR/ERK pathway and regulates T678 phosphorylation. We also show that TNF-α enhances epithelial wound healing through TACE, ERK, and GEF-H1. Taken together, our findings can explain the mechanisms leading to hierarchical activation of Rac and RhoA by TNF-α through a single GEF. This mechanism could coordinate GEF functions and fine-tune Rac and RhoA activation in epithelial cells, thereby promoting complex functions such as sheet migration.

RhoA and Rho kinase mediate cyclosporine A and sirolimus-induced barrier tightening in renal proximal tubular cells.

The regulation and maintenance of the paracellular transport in renal tubular epithelia is vital for kidney functions. Combination of the immunosuppressant drugs cyclosporine A (CsA) and sirolimus (SRL) exerts powerful immunosuppression, but also causes nephrotoxicity. We have previously shown that CsA and SRL elevate transepithelial resistance (TER) in kidney tubular cells partly through MEK/ERK1/2. In this work we examined the hypothesis that the RhoA pathway may also be mediating effects of CsA and SRL. We show that CsA and the CsA/SRL combination activated RhoA, induced cofilin phosphorylation and promoted stress fiber generation. The Rho kinase (ROK) inhibitor, Y27632, prevented CsA and CsA/SRL-induced cofilin phosphorylation and actin remodelling, reduced the TER increase and prevented the rise in claudin-7 levels caused by the drugs. Expression of the exchange factor GEF-H1/lfc was elevated in cells treated with CsA and CsA/SRL. GEF-H1 silencing inhibited RhoA activation by ≈50%, and potently reduced cofilin phosphorylation and stress fiber formation induced by CsA and CsA/SRL. However, GEF-H1 downregulation did not prevent the TER change. Thus the Rho/Rho kinase pathway was involved in mediating CsA and CsA/SRL-induced cytoskeleton rearrangement and TER changes via claudin-7 expression. Our data however point to differential regulation of Rho activation involved in central cytoskeleton remodelling, that is GEF-H1-dependent and junctional permeability that does not require GEF-H1.